Seasonal Changes in the Physiology of S24 Perennial Ryegrass (Lolium perenne L.). 2. Potential Leaf and Canopy Photosynthesis during the Transition from Vegetative to Reproductive growth

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Abstract
The photosynthetic potential of successive youngest fully-expanded leaves of S24 L. perenne, grown as simulated swards under natural environmental conditions, was measured during establishment in autumn, over winter and during the transition from vegetative to reproductive growth the following spring. Measurements were made at a standard light energy receipt of 250 J m−2 s−1 (400–700 nm) and at 15 °C. The photosynthetic potential of the leaves decreased in autumn as the swards increased in density under worsening environmental conditions. During the spring, photosynthetic rates rose from low over-winter values so that by March, before stem elongation began, they were equal to the rates in the previous autumn. Following stem elongation there was a further increase in leaf potential. Reasons for these changes in leaf potential are discussed. During spring, the photosynthetic potential of the canopy also rose - both as measured, and as predicted by the Monteith model of canopy photosynthesis. Use of the model suggested that increasing leaf potential made the greatest contribution to the rise in the potential of the canopy, although, following stem elongation, changes in LAI and canopy structure had a further significant effect.